Power plants can have a boiler that generates steam that is expanded in a steam turbine. The boiler further generates flue gas that is treated to remove impurities before the flue gas is vented into the atmosphere.
In order to reduce carbon dioxide emission into the atmosphere (carbon dioxide has a negative environmental impact), the boiler can burn a fossil fuel with pure or substantially pure oxygen, in presence of recirculated flue gas (oxy fuel combustion). Oxyfuel combustion allows generation of flue gas having a high content of carbon dioxide, to make it easier carbon dioxide separation and sequestration.
Since the flue gas has high content of carbon dioxide, the flue gas treatment systems downstream of the boiler must be so designed in order to prevent flue gas contamination with air or other gases, because this would dilute the carbon dioxide and would increase the effort for carbon dioxide separation.
Often the fossil fuel used in boilers (e.g. coal, but also other fossil fuels, such as oil) contains sulfur that during combustion generates SO2, SO3 and other sulphur compounds. These compounds can give corrosion problems (within the power plant) or acid rains (if vented) and must thus be removed from the flue gas.
Often, sulphur compounds are removed from the flue gas by reacting them with lime and/or limestone, generating calcium sulfite CaSO3; calcium sulfite is then oxidized into calcium sulfate CaSO4 (gypsum) with air; calcium sulfate is then removed.
Oxidation of calcium sulfite into calcium sulfate must be done preventing oxygen from contaminating the flue gas.
U.S. Pat. No. 8,337,790 discloses a system for sulphur removal having an absorber and a separate container with an oxidation chamber and a buffer chamber connected via an overflow. During operation the reaction chamber receives a suspension containing calcium sulfite CaSO3 from the absorber; air is supplied into the reaction chamber to oxidize the calcium sulfite into calcium sulfate CaSO4 that precipitates and is removed from the oxidation chamber. The sulphur free suspension passes to the buffer chamber by overcoming the overflow and is fed back to the absorber via a pump.
Since the reaction chamber and the buffer chamber are defined in one and the same tank, the air supplied into the reaction chamber is also above the suspension contained in the buffer chamber; this causes risks that air bubbles reach the absorber.